Technical info

Technical info

Type J

TYPE J

TYPE J THERMOCOUPLE (Iron/Constantan)

TypeJColorCode
Composed of a positive leg which is iron and a negative leg which is approximately 45 % nickel-55% copper.
(Note – Constantan is Copper-Nickel.)

When protected by compacted mineral insulation and appropriate outer sheath, Type J is useable from 0 to 816°C, (32 to 1500°F). It is not susceptible to aging in the 371 to 538°C, (700 to 1000°F) temperature range. A drift rate of 1 to 2°C, (2 to 4°F) occurs with Type E and K in the 371 to 538°C, (700 to 1000°F) temperature range. This low cost, stable calibration is primarily used with 96% pure MgO insulation and a stainless steel sheath.
Thermocouple Grade- 32°F to 1382°F, 0 to 750°C
Extension Grade- 32°F to 392°F, 0 to 200°Ci
Thermocouple Colour Code

Type K

Type K

TYPE K THERMOCOUPLE (Chromel / Alumel)200°C to +1260°C / -328°F to +2300°F

Chromel {90% nickel and 10% chromium} Alumel {95% nickel, 2% manganese, 2% aluminium and 1% silicon}

Thermocouple Type K

  • This is the most common thermocouple type that provides the widest operating temperature range. Type K thermocouples generally will work in most applications because they are nickel based and have good corrosion resistance.
  • Positive leg is non-magnetic (Yellow), negative leg is magnetic (Red).
  • Traditional base-metal choice for high temperature work.
  • Appropriate for use in oxidizing or inert atmospheres at temperatures up to 1260°C (2300°F).
  • Vulnerable to sulfur attack (refrain from exposing to sulfur-containing atmospheres).
  • Perform best in clean oxidizing atmospheres.
  • Not recommended for use under partially oxidizing conditions in vacuum, or when subjected to alternating cycles of oxidization and reduction.
TypeKthermocouplecolor

Composed of a positive leg, which is approximately 90% nickel, 10% chromium and a negative leg, which is approximately 95% nickel, 2% aluminum, 2% manganese and 1% silicon.Type K Thermocouples are the most common general purpose thermocouple with a sensitivity of approximately 41µV/°C, chromel positive relative to alumel. It is inexpensive, and a wide variety of probes are available in its -200°C to +1260°C / -328°F to +2300°F range. Type K was specified at a time when metallurgy was less advanced than it is today, and consequently characteristics vary considerably between samples. One of the constituent metals, nickel, is magnetic; a characteristic of thermocouples made with magnetic material is that they undergo a step change in output when the magnetic material reaches its Cure Point (around 354 °C for type K thermocouples).
Type K Thermocouple ( Chromel/Constantan)

Type K thermocouples usually work in most applications as they are nickel based and exhibit good corrosion resistance. It is the most common sensor calibration type providing the widest operating temperature range. Due to its reliability and accuracy the Type K thermocouple is used extensively at temperatures up to 2300°F (1260°C). This type of thermocouple should be protected with a suitable metal or ceramic protection tube, especially in reducing atmospheres. In oxidizing atmospheres, such as electric furnaces, tube protection is not always necessary when other conditions are suitable; however, it is recommended for cleanliness and general mechanical protection. Type K will generally outlast Type J because the JP wire rapidly oxidizes, especially at higher temperatures.

Temperature Range:

  • Thermocouple grade wire, −454° to 2,300°F (−270 to 1,260°C)
  • Extension grade wire, −32° to 392°F (0 to 200°C)
  • Melting Point, 2550°F (1400°C

Accuracy (whichever is greater):

  • Standard: ± 2.2C% or ±.75%
  • Special Limits of Error: ± 1.1C or 0.4%

Deviations in the alloys can affect the accuracy of thermocouples. For type K thermocouples the tolerance class one is given as ± 1.5 K between -40 and 375 °C. However, deviations between thermocouples coming from the same production are very small and a much higher accuracy can be achieved by individual calibration.

Metallurgical changes can cause a calibration drift of 1 to 2°C in a few hours, increasing to 5 °C over time. A special grade of Type K is available that can maintain special limit accuracy up to ten times longer than the regular grade.

Type E

TYPE E

TYPE E THERMOCOUPLE (Chromel / Constantan)

TypeEColorCodeComposed of a positive leg, which is approximately 90% nickel, 10 chromium and a negative leg, which is approximately 95% nickel, 2% aluminum, 2% manganese and 1% silicon.

When protected by compacted mineral insulation and appropriate outer sheath, Type E is usable from 0 to 900°C, (32 to 1652°F). This Thermocouple has the highest EMF output per degree of all recognized thermocouples. If the temperature is between 316 to 593°C, (600 to 1100°F), we recommend using type J or N because of aging which can cause drift of 1 to 2°C, (2 to 4°F) in a few hours time. For applications below 0°C, (32°F), special selection of alloys are usually required.

Type T

TYPE T

TYPE T THERMOCOUPLE (Copper / Constantan)

TypeTColorCode
(copper-constantan) type T thermocouples are suited for measurements in the −200 to 350 °C range. Often used as a differential measurement since only copper wire touches the probes

When protected by compacted mineral insulation and appropriate outer sheath, Type T is usable from 0 to 350°C, (32 to 662°F). Type T is very stable and is used in a wide variety of cryogenic and low temperature applications. For applications below 0°C, (32°F) special selection of alloys are usually required.

Type N

TYPE N

TYPE N THERMOCOUPLE (Nicrosil / Nisil)

TypeNColorCode
When protected by compacted mineral insulation and appropriate outer sheath, Type N is useable from 0 to 1260°C, (32 to 2300°F). Type N was developed to overcome several problems inherent in Type K thermocouples. Aging in the 316 to 593°C, (600 to 1100°F) temperatures is considerably less. Type N has also been found to be more stable than Type K in nuclear environments.

Type N (Nicrosil?Nisil) (nickel-chromium-silicon/nickel-silicon) thermocouples are suitable for use between -270 °C and 1300 °C owing to its stability and oxidation resistance. Sensitivity is about 39 ?V/°C at 900 °C, slightly lower compared to type K.

Type B

TYPE B

TYPE B THERMOCOUPLE (Platinum / Rhodium)

TypeNColorCode

Composed of a positive leg which is approximately 14% chromium, 1.4% Silicon and 84.6% Nickel, a negative leg which is approximately 4.4% Silicon, 95.6% Nickel.

When protected by compacted mineral insulation and appropriate outer sheath, Type B is usable from 871 to 1704°C, (1600 to 3100°F). Also easily contaminated, and damaged by reducing atmospheres. The same protective measures as shown above apply to type B Thermocouples.

Type S

TYPE S

TYPE S THERMOCOUPLE (Platinum / Rhodium)

TypeSColorCode

Composed of a positive leg which is approximately 70% Platinum, 30% Rhodium and a negative leg which is approximately 94% Platinum, 6% Rhodium.

When protected by compacted mineral insulation and appropriate outer sheath, Type S is usable from 0 to 1482°C, (32 to 2700°F). Easily contaminated. Reducing atmospheres are particularly damaging. Type S should be protected with gas tight ceramic tubes, a secondary tube of porcelain and silicon carbide or metal outer tubes, as conditions require.

Type R

TYPE R

TYPE R THERMOCOUPLE (Platinum / Rhodium)

TypeRColorCode

Composed of a positive leg which is approximately 70% Platinum, 30% Rhodium and a negative leg which is approximately 94% Platinum, 6% Rhodium.

When protected by compacted mineral insulation and appropriate outer sheath, Type R is usable from 0 to 1482°C, ( 32 to 2700°F).Type R has a higher EMF output than type S. Also easily contaminated, and damaged by reducing atmospheres. Type R should by protected in a similar fashion as Type S.

Technical

Technical

Thermocouple Types Calibration types have been established by the American Society for Testing and Materials(ASTM)according to their temperature versus EMF characteristics in accordance with ITS-90,in standard or special tolerances.

Thermocouple_Types_Picture
Thermocouple Millivolts/Temperature Curves Also,calibration types are designed to deliver as close to a straight line voltage curve inside their temperature application range as possible. This makes it easier for an instrument or temperature controller to correctly correlate the received voltage to a particular temperature.
Thermocouple_Milivolts_Chart2

Thermocouple Response TimeThe smaller the diameter,the faster the thermocouple responds. Grounding the junction also improves response time by approximately 50 percent based on the sensor achieving 63.2 percent of the final reading or to the first time constant. It takes approximately five time constants to obtain steady state readings.

Thermal_Response_Time

Thermocouple TheoryA thermocouple circuit has at least two junctions: the measurement junction and a reference junction. Typically, the reference junction is created where the two wires connect to the measuring device. This second junction it is really two junctions: one for each of the two wires, but because they are assumed to be at the same temperature (isothermal) they are considered as one (thermal) junction.

thermocouple_diagram-1

Thermopile Multi-SensorA thermopile is a thermoelectric device that consists of an array of thermocouples connected in series. It is widely used in non-contact temperature measurement applications and temperature monitoring systems. Thermopiles detect the temperature of an object by absorbing the infared (IR) radiation that emits from the object’s surface. Most of the thermopile detectors are equipped with a black body surface for effectively absorbing the IR radiation.

thermopile

Reference

Reference

Advantages Of Thermocouples

  • Capable of being used to directly measure temperatures up to 2600 °C.
  • The thermocouple junction may be grounded and brought into direct contact with the material being measured.

Aging Of Thermocouples

Thermoelements are often used at high temperatures and in reactive furnace atmospheres. In this case the practical lifetime is determined by aging. The thermoelectric coefficients of the wires in the area of high temperature change with time and the measurement voltage drops.

The simple relationship between the temperature difference of the joints and the measurement voltage is only correct if each wire is homogeneous. With an aged thermocouple this is not the case. Relevant for the generation of the measurement voltage are the properties of the metals at a temperature gradient. If an aged thermocouple is pulled partly out of the furnace, the aged parts from the region previously at high temperature enter the area of temperature gradient and the measurement error is significantly increased. However an aged thermocouple that is pushed deeper into the furnace gives a more accurate reading.

Thermocouple Power Production

A thermocouple can produce current, which means it can be used to drive some processes directly, without the need for extra circuitry and power sources. For example, the power from a thermocouple can activate a valve when a temperature difference arises. The electrical energy generated by a thermocouple is converted from the heat which must be supplied to the hot side to maintain the electric potential. A continuous flow of heat is necessary because the current flowing through the thermocouple tends to cause the hot side to cool down and the cold side to heat up (the Peltier effect).

Thermocouples can be connected in series to form a thermopile, where all the hot junctions are exposed to a higher and all the cold junctions to a lower temperature. The output is the sum of the voltages across the individual junctions, giving larger voltage and power output. Using the radioactive decay of transuranic elements as a heat source, this arrangement has been used to power spacecraft on missions too far from the Sun to utilize solar power.

Thermocouple Voltage-Temperature Relationship

For typical metals used in thermocouples, the output voltage increases almost linearly with the temperature difference (?T) over a bounded range of temperatures. For precise measurements or measurements outside of the linear temperature range, non-linearity must be corrected. The nonlinear relationship between the temperature difference (?T) and the output voltage (mV) of a thermocouple can be approximated by a polynomial:

The coefficients an are given for n from 0 to between 5 and 13 depending upon the metals. In some cases better accuracy is obtained with additional non-polynomial terms[4]. A database of voltage as a function of temperature, and coefficients for computation of temperature from voltage and vice-versa for many types of thermocouple is available online.

In modern equipment the equation is usually implemented in a digital controller or stored in a look-up table; older devices use analog circuits. Piece-wise linear approximations are an alternative to polynomial corrections.